1. Field of the Invention
The present invention relates generally to lithium-ion batteries and, more particularly, to a method for extending the life of Lithium-Ion (Li-ion) batteries of the type used in spacecraft by optimizing the state-of-charge and battery temperature throughout the on-orbit battery charging period.
2. Description of the Prior Art
Lithium batteries have existed for years, mainly as primary (non-rechargeable) types in the form of small "coin" cells. Larger primary cells are considered hazardous materials and so are not widely available in the United States. Lithium is a very reactive element which is desirable for use in batteries but is dangerous because its reactivity makes it potentially flammable.
For normal shipping, the U.S. Department of Transportation limits the amount of lithium in a single cell to 1 g. Solid-electrolyte lithium cells (lithium-iodine and lithium-magnesium-dioxide types, for example) have high internal impedances. This limits their use to products like pacemakers and other low-current, long-life applications. Liquid-cathode lithium cells can be discharged at a higher rate, but these types are generally limited to memory-retention and battery-backup applications.
Rechargeable (secondary) lithium batteries appeared in the 1980s. These batteries use lithium metal as the negative electrode (the anode) and an "intercalation" positive electrode (the cathode). Intercalation refers to an electrochemical reaction in which ions bond to the cathode material. Because this reaction is reversible (de-intercalation), the battery can be made rechargeable.
When a rechargeable lithium battery discharges, the lithium metal gives off ions to the electrolyte, which is either a liquid or a solid polymer. These lithium ions migrate to the cathode and ionically bond with the material. The main problem with this battery type resides in dendrites--small fingers of lithium metal that form while the battery is charging. Dendrites increase the metal's surface area, producing a greater reactivity with the electrolyte. Thus, the battery becomes increasingly sensitive to abuse because the number of dendrites increases with each charge-discharge cycle.
To overcome the problems associated with lithium metal in batteries, researchers experimented with the use of intercalation materials for both the anode and cathode, producing a component known as the lithium-ion (Li-ion) battery. Lithium metal is not present; instead, positively charged lithium ions travel from cathode to anode during charge and from anode back to cathode during discharge. This back and forth ion flow during the charge and discharge cycles has led to the expressions of "swing" and "rocking-chair" batteries.
The use of intercalation electrodes not only eliminates the need for lithium metal, but also simplifies manufacturing because manufacturers can construct the battery at zero potential. The manufacturer can then charge the battery after assembly, thereby reducing the possibility of damage due to short circuits.
Lithium ion batteries are rapidly becoming the power source of choice for space applications. They exhibit high energy and power both per unit volume and per unit weight in comparison with NiCd, nickel-metal hydride (NiMH), and other rechargeable types.
Because of one of their unique operating characteristics, lithium ion battery cells require careful charge management to ensure that significant over charge and over discharge does not occur. This is for the reason that lithium ion batteries possess an extreme sensitivity to overcharging and over-discharging not found in most other types of batteries. Such charge management may be achieved by limiting the maximum voltage to which the cell is charged. In order to achieve the maximum possible energy stored in the cell while limiting the over charge and over discharge, a device is required that controls the voltage. Also, in many applications, if a cell opens, then the whole battery would be lost. It is desirable to allow the cell to be completely bypassed if it fails in this manner. The ability to monitor temperature and adjust the maximum charge voltage accordingly is also desirable. A feature to allow varying the charge voltage set point from outside the device is also desired.
Lithium-Ion batteries are normally charged at constant current to the end-of-charge voltage limit in some cases (i.e. the two-step charge method), the charging current is then stepped to a lower value, and charging is resumed until the battery returns to the end-of-charge voltage limit. This is sometimes referred to as the constant current-constant current method. Alternately after reaching the cut-off limit, the battery can be held at the end-of-charge voltage while the current decreases as needed to maintain the constant voltage. This is sometimes referred to as the constant current-constant voltage method. Battery charging and discharging normally occurs at a nominal design temperature with no attempt to control or adjust battery temperatures during the charge/discharge process.
The present invention relates to optimizing battery charging and discharging temperatures, specifically, cold charging in the range of about +5.degree. C. to -20.degree. C. and warm discharging in the range of about +5.degree. C. to +30.degree. C. Also proposed is the technique of partially charging the battery and then waiting as long as possible before completing the charge in order to minimize the time at high battery voltage.
Of interest in this regard is commonly assigned U.S. Pat. No. 5,395,706 issued Mar. 7, 1995 to John C. Hall. The Hall patent discloses a method of operating a nickel-hydrogen battery in a manner which serves to increase its charge capacity. That method comprises the step of completing the recharging process for the battery at a temperature T.sub.1, in the range of approximately -10.degree. C. down to -30.degree. C. which is lower than a temperature T.sub.2, in the range of approximately -10.degree. C. to +5.degree. C., at which discharge customarily begins. At the onset of the recharge operation the temperature may be in the range of +25.degree. C. to +40.degree. C. However, as recharge proceeds, the temperature is caused to fall to the range of -10.degree. C. to -30.degree. C. which is optimum for full recharge. The temperature T.sub.1 is chosen to maximize the extent of the reaction represented by the equation: EQU Ni(OH).sub.2 +OH.sup.- =NiOOH+H.sub.2 O+e.sup.-
versus the reaction represented by the equation: EQU 2OH.sup.- =1/2O.sub.2 +H.sub.2 O+2e.sup.-
Subsequently, as recited in the patent, it is desirable to heat the battery to the temperature T.sub.2 in readiness for discharge. A preferred recharge temperature is less than approximately -10.degree. C. The battery includes a positive electrode which may include electrochemically active Ni(OH).sub.2 (possibly mixed with Co(OH).sub.2) and electrically conductive material having a resistivity less than approximately 0.1 ohm-cm, a negative electrode which is of a material which catalyzes the oxidation and reduction of H.sub.2, and an electrolyte which is a solution of KOH (typically 20% to 40% by weight).
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.